Proteins that selectively bind to selected targets by way of non-covalent interaction play a crucial role as reagents in biotechnology, medicine, bioanalytics as well as in the biological and life sciences in general. Antibodies, i.e. immunoglobulins, are a prominent example of this class of proteins. Despite the manifold needs for such proteins in conjunction with recognition, binding and/or separation of ligands/targets, almost exclusively immunoglobulins are currently used. The application of other proteins with defined ligand-binding characteristics, for example the lectins, has remained restricted to special cases.
Additional proteinaceous binding molecules that have antibody-like functions are the members of the lipocalin family, which have naturally evolved to bind ligands. Lipocalins occur in many organisms, including vertebrates, insects, plants and bacteria. The members of the lipocalin protein family (Pervaiz, S., & Brew, K. (1987) FASEB J. 1, 209-214) are typically small, secreted proteins and have a single polypeptide chain. They are characterized by a range of different molecular-recognition properties: their ability to bind various, principally hydrophobic molecules (such as retinoids, fatty acids, cholesterols, prostaglandins, biliverdins, pheromones, tastants, and odorants), their binding to specific cell-surface receptors and their formation of macromolecular complexes. Although they have, in the past, been classified primarily as transport proteins, it is now clear that the lipocalins fulfill a variety of physiological functions. These include roles in retinol transport, olfaction, pheromone signaling, and the synthesis of prostaglandins. The lipocalins have also been implicated in the regulation of the immune response and the mediation of cell homoeostasis (reviewed, for example, in Flower, D. R. (1996) Biochem. J. 318, 1-14 and Flower, D. R. et al. (2000) Biochim. Biophys. Acta 1482, 9-24).
The lipocalins share unusually low levels of overall sequence conservation, often with sequence identities of less than 20%. In strong contrast, their overall folding pattern is highly conserved. The central part of the lipocalin structure consists of a single eight-stranded anti-parallel β-sheet closed back on itself to form a continuously hydrogen-bonded β-barrel. This β-barrel forms a central cavity. One end of the barrel is sterically blocked by the N-terminal peptide segment that runs across its bottom as well as three peptide loops connecting the β-strands. The other end of the β-barrel is open to the solvent and encompasses a target-binding site, which is formed by four flexible peptide loops. It is this diversity of the loops in the otherwise rigid lipocalin scaffold that gives rise to a variety of different binding modes each capable of accommodating targets of different size, shape, and chemical character (reviewed, e.g., in Flower, D. R. (1996), supra; Flower, D. R. et al. (2000), supra, or Skerra, A. (2000) Biochim. Biophys. Acta 1482, 337-350).
International patent application WO 99/16873 discloses polypeptides of the lipocalin family with mutated amino acid positions in the region of the four peptide loops, which are arranged at the end of the cylindrical β-barrel structure encompassing the binding pocket, and which correspond to those segments in the linear polypeptide sequence that includes the amino acid positions 28 to 45, 58 to 69, 86 to 99, and 114 to 129 of the bilin-binding protein of Pieris brassicae. Members of the lipocalin familiy have been reported to be post-translationally modified, e.g. phosphorylation and glycosylation of tear lipocalin (e.g. You, J., et al. (2010) Electrophoresis 31, 1853-1861). Nevertheless no post-translational modification is required for their molecular recognition properties.
International patent application WO 00/75308 discloses muteins of the bilin-binding protein, which specifically bind digoxigenin, whereas the international patent applications WO 03/029463 and WO 03/029471 relate to muteins of the human neutrophil gelatinase-associated lipocalin (hNGAL) and apolipoprotein D, respectively. In order to further improve and fine tune ligand affinity, specificity as well as folding stability of a lipocalin variant various approaches using different members of the lipocalin family have been proposed (Skerra, A. (2001) Rev. Mol. Biotechnol. 74, 257-275; Schlehuber, S., and Skerra, A. (2002) Biophys. Chem. 96, 213-228), such as the replacement of additional amino acid residues. The PCT publication WO 2006/56464 discloses muteins of human neutrophile gelatinase-associated lipocalin with binding affinity for CTLA-4 in the low nanomolar range.
International patent application WO 2005/19256 discloses muteins of tear lipocalin with at least one binding site for different or the same target ligand and provides a method for the generation of such muteins of human tear lipocalin. According to this PCT application, certain amino acid stretches within the primary sequence of tear lipocalin, in particular the loop regions that include amino acids 7-14, 24-36, 41-49, 53-66, 69-77, 79-84, 87-98, and 103-110 of mature human tear lipocalin, are subjected to mutagenesis in order to generate muteins with binding affinities. The resulting muteins have binding affinities for the selected ligand (KD) in the nanomolar range, in most cases >100 nM. International patent application WO 2008/015239 discloses muteins of tear lipocalin binding a given non-natural ligand, including the IL-4 receptor alpha. Binding affinities are in the nanomolar range, reaching as low as almost 1×10−10 M in surface plamon resonance experiments.
Human tear lipocalin (TLPC or Tlc), also termed lipocalin-1, tear pre-albumin or von Ebner gland protein, was originally described as a major protein of human tear fluid (approximately one third of the total protein content) but has also been identified in several other secretory tissues including prostate, adrenal gland, thymus, mammary gland, testis, nasal mucosa and tracheal mucosa as well as corticotrophs of the pituitary gland. Homologous proteins have been found in rhesus monkey, chimpanzee, rat, mouse, pig, hamster, cow, dog and horse. Tear lipocalin is an unusual lipocalin member in that it exhibits an unusually broad ligand specificity, when compared to other lipocalins, and in its high promiscuity for relative insoluble lipids (see Redl, B. (2000) Biochim. Biophys. Acta 1482, 241-248). This feature of tear lipocalin has been attributed to the protein's function in inhibiting bacterial and fungal growth at the cornea. A remarkable number of lipophilic compounds of different chemical classes such as fatty acids, fatty alcohols, phospholipids, glycolipids and cholesterol are endogenous ligands of this protein. Interestingly, in contrast to other lipocalins the strength of ligand (target) binding correlates with the length of the hydrocarbon tail both for alkyl amides and fatty acids. Thus, tear lipocalin binds most strongly the least soluble lipids (Glasgow, B. J. et al. (1995) Curr. Eye Res. 14, 363-372; Gasymov, O. K. et al. (1999) Biochim. Biophys. Acta 1433, 307-320). The 1.8-Å crystal structure of tear lipocalin revealed an unusually large cavity inside its β-barrel (Breustedt, D. A. et al. (2005) J. Biol. Chem. 280, 1, 484-493).
Despite this progress it would be still desirable to have a human tear lipocalin mutein that has improved binding properties for IL-4 receptor alpha, in particular of higher binding affinity, simply for the reason to further improve the suitability of muteins of human tear lipocalin in diagnostic and therapeutic applications.